A sheet processing apparatus including: a stack tray on which a sheet successively delivered from a delivery port is stacked, the stack tray including: a folding route receiving the sheet from the delivery port; a sheet folding device provided in the folding route to fold the sheet; and a sheet stacking surface for stacking the sheet, wherein the sheet processing apparatus is operable in selectively between a first mode in which a sheet from the delivery port is stacked on the sheet stacking surface, and a second mode in which a sheet folded by the sheet folding device is stacked on the same sheet stacking surface as in the first mode.